Add "get_workers" method to "QuotaDriverAPI" class

This method returns the quota driver workers that needs to be spawned
during the plugin initialization.

Change-Id: Id9840912b9d0018d008b6961d24dadbfaafc9f8e
Related-Bug: #1954662

neutron_lib/db/quota_api.py

@@ -167,3 +167,16 @@ class QuotaDriverAPI(object, metaclass=abc.ABCMeta):
         :return: None if passed; ``OverQuota`` if quota limits are exceeded,
                  ``InvalidQuotaValue`` if delta values are invalid.
         """
+
+    @staticmethod
+    @abc.abstractmethod
+    def get_workers():
+        """Return the quota driver workers to be spawned during initialization
+
+        This method returns the quota driver workers that needs to be spawned
+        during the plugin initialization. For example, ``DbQuotaNoLockDriver``
+        requires a ``PeriodicWorker`` to clean up the expired reservations left
+        in the database.
+
+        :return: list of ``worker.BaseWorker`` or derived instances.
+        """

releasenotes/notes/quota-driver-api-get-workers-f540a81235dbf48d.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Added abstract method ``get_workers`` to ``QuotaDriverAPI`` metaclass. This
+    method returns the quota driver workers that needs to be spawned during the
+    plugin initialization.